Eenie, meenie, miney, or mo?

Four rules ot thumb for selecting the right application

Horizontal directional drilling uses a computer-guided boring head to open a “lubricated” underground conduit into which a pipe can be pulled.

Good for: Pipes that don't have strict line and grade requirements, and with few service leads.

Not so good for: Drilling through hardpan or cobble, or where multiple setups are necessary.

Pipe bursting uses the old pipe itself as a conduit, opening it up with a bursting head and expander to admit the new pipe, which is often larger in diameter than the old pipe. Generally, engineers recommend not going up more than one pipe size, but depending on the soil, the diameter of the new pipe can be more than twice the original pipe's.

Good for: Areas with poor access, and where the existing pipe is at the desired line and grade.

Not so good for: Applications where the existing pipe has experienced significant line or grade distortions, or where many leads need to be connected.

Microtunneling involves pushing a pipe into a conduit opened by a remotely controlled boring machine employing a cutting head at the face.

Good for: 36-inch-diameter and larger pipes where excavating is impractical and the soil conditions are permissive.

Not so good for: Departments with small budgets. Usually chosen when open-cut excavation isn't feasible because of soils or sensitive surface features.